are trekking bars popular for winter use?
I see mostly flat bars but assume thats because they are the easiest to come across... yes no?

I don't know about the popularity of trekking bars, I do know that I like them. I tried about six different styles/types of bars with and without various bar ends before I found them. I believe that mine are mounted in a reversed position.

I just installed some on my winter/cargo/touring bike yesterday. So far they're comfortable but the handling doesn't feel as steady as it did with the On-One Midge bars (off-road drop bars) I was using before this. The main problem is that the high leverage position puts you further from your brakes than I'd like. If your roads are kept reasonably clear they'd be fine, but for rutted, unplowed snow like we have here you'd probably be better off with straight bars. Personally, I'll probably be swapping the Midges back on next winter, but for summer usage I think the trekking bars will be great.

I installed Trekking (butterfly) bars myself for this season commuting. I love them so far. I also use aero bars as well, as far as brake & shifter location is concerned I guess it depends on where you locate them on the bar. I mounted mine on the lead bar so the brakes are near the aero bars as well as access from anywhere on the Trekking bar.